Technical Field
This disclosure relates to systems and methods for maintaining a spatial relationship between a tool of a multi-axis machine (e.g., a fluid jet nozzle of a fluid jet cutting machine) and a workpiece to be processed by the tool. The disclosure also relates to systems and methods for sensing collisions with an obstruction in the controlled path of the tool and adjusting operation of the machine accordingly.
Description of the Related Art
High-pressure fluid jets, including high-pressure abrasive waterjets, are used to cut a wide variety of materials in many different industries. Systems for generating high-pressure abrasive waterjets are currently available, such as, for example, the Mach4™ five-axis abrasive waterjet system manufactured by Flow International Corporation, the assignee of the present invention, as well as other systems that include a cutting head assembly mounted to an articulated robotic arm or other motion system. Other examples of abrasive fluid jet cutting systems are shown and described in Flow's U.S. Pat. No. 5,643,058, which is incorporated herein by reference. The terms “high-pressure fluid jet” and “jet” should be understood to incorporate all types of high-pressure fluid jets, including but not limited to high-pressure waterjets and high-pressure abrasive waterjets. In such systems, high-pressure fluid, typically water, flows through an orifice of an orifice unit in a cutting head to form a high-pressure jet, into which abrasive particles may be combined as the jet flows through a mixing chamber and a mixing tube to form a high-pressure abrasive waterjet. The high-pressure abrasive waterjet is typically discharged from the mixing tube and directed toward a workpiece to cut the workpiece along a designated path.
Various systems are currently available to move a high-pressure fluid jet along a designated path. Such systems may commonly be referred to as, for example, three-axis and five-axis machines. Conventional three-axis machines mount the cutting head assembly in such a way that it can move along an x-y plane and perpendicularly thereto along a z-axis, namely toward and away from the workpiece. In this manner, the high-pressure fluid jet generated by the cutting head assembly is moved along the designated path in an x-y plane, and is raised and lowered relative to the workpiece, as may be desired. Conventional five-axis machines work in a similar manner but provide for movement about two additional non-parallel rotary axes. Other systems may include a cutting head assembly mounted to an articulated robotic arm, such as, for example, a six-axis robotic arm which articulates about six separate rotary axes.
Computer-aided manufacturing (CAM) processes may be used to drive or control such conventional machines along a designated path, such as by enabling two-dimensional or three-dimensional models of workpieces generated using computer-aided design (i.e., CAD models) to be used to generate code to drive the machines. For example, a CAD model may be used to generate instructions to drive the appropriate controls and motors of the machine to manipulate the machine about its translational and/or rotary axes to cut or process a workpiece as reflected in the model.
During the fluid jet cutting process, dimensional accuracy and cut quality may be dependent on, among other things, precisely maintaining a desired distance between the end of the nozzle or mixing tube and the surface of the workpiece being cut, often referred to as the standoff distance. Maintaining a precise standoff distance becomes particularly important as fluid jet cutting technology advances from flat-stock 2-D cutting, to applications involving curved material, beveled cuts and other complex cutting profiles enabled by five-axis and other multi-axis control.
Historically, for example, commanded five-axis motion control of 2-D flat stock cutting is based on compound angle calculations evaluated based on the inferred (“nominal”) distance between the end of the nozzle or mixing tube and the surface of the workpiece to be cut. In reality, the actual standoff distance will deviate from the nominal distance, for example, warping due to stress relieving of material during the cut, natural material bow, or the “as provided” state from a manufacturer will introduce error into any cut edge off of the vertical axis. This error is particularly apparent and undesirable as the cut edge shifts further from vertical, for example, in an intentional bevel cut. The example contour follower apparatus and related systems and methods described herein help to ensure that the distance between the focal point of the machine is known relative to the surface of the workpiece being cut. This allows the controller to hold the machine focal point on the surface of the workpiece despite any deviations in the terrain of the workpiece and provides enhanced functionality over prior standoff distance control systems and methodologies, such as those shown and described in Flow's U.S. Pat. No. 7,331,842. For example, the example contour follower apparatus and related systems and methods provide, among other things, enhanced accuracy with which the standoff distance is maintained, including when cutting at particularly steep angles, such as when making an intentional bevel cut.